A zirconia sintered body having a small amount of Y2O3 solid-solubilized as a stabilizing agent (hereinafter referred to as a “partially stabilized zirconia sintered body”) has high strength and high toughness. Thus, such a partially stabilized zirconia sintered body is utilized for a mechanical structural material, such as a cutting tool, a die, a nozzle or a bearing. Further, it is utilized as a biomaterial such as a dental material, other than the mechanical structural material. In a case where a partially stabilized zirconia sintered body is to be used as a dental material, not only mechanical properties such as high strength and high toughness, but also optical characteristics such as translucency, color tone, etc. from the aesthetic point of view, are required.
From the aesthetic point of view, as zirconia having a translucency, a zirconia single crystal (cubic zirconia) containing about 10 mol % of yttria, has been utilized in e.g. ornaments. However, the zirconia single crystal has had a problem that its strength is extremely low.
On the other hand, a zirconia sintered body being a polycrystalline body of zirconia has no translucency. As a cause for this, it is known that pores existing in crystal grains and between crystal grains cause light scattering. Therefore, a study has heretofore been made to impart a translucency to a polycrystalline zirconia sintered body by reducing the pores, i.e. by increasing the sintered body density.
For example, Patent Document 1 discloses a zirconia sintered body which contains from 2 to 4 mol % of yttria and has an alumina content of at most 0.2 wt % and which has a total light transmittance of at least 35% at a thickness of 1 mm. However, the sintered body disclosed in Examples had a total light transmittance of 41%, which was 36% as a total light transmittance to light with a wavelength of 600 nm at a thickness of 1.0 mm. The sintered body was a sintered body having sufficient translucency and strength to be used as a denture for back tooth. On the other hand, the sintered body had a problem that the translucency was insufficient for use as a denture for front tooth.
Patent Document 2 discloses a zirconia sintered body containing from 1.5 to 5 mol % of yttria and having a porosity of at most 0.6%. However, the sintered body is a zirconia sintered body obtained by pressure sintering using hot isostatic pressing (hereinafter referred to as “HIP”), and with a zirconia sintered body obtained by pressureless sintering, sufficient translucency was not obtained.
Further, Patent Document 3 discloses a zirconia sintered body containing more than 4 mol % and at most 7 mol % of yttria, and having a total light transmittance of at least 40% at a wavelength of 600 nm at a thickness of 1 mm. This sintered body is also a zirconia sintered body obtained by pressure sintering using HIP, and with a zirconia sintered body obtained by pressureless sintering, sufficient translucency was not obtained.
Non-Patent Document 1 discloses a zirconia sintered body having transparency obtained by spark plasma sintering (hereinafter referred to as “SPS”) of a zirconia powder containing 3 mol % of yttria and 8 mol % of yttria.
However, in order to use the zirconia sintered body disclosed in Patent Document 3 or Non-Patent Document 1 as a denture for front tooth, the transparency was too high and unnatural.
Further, in order to prepare a denture from a zirconia sintered body having a translucency, a method is known wherein a provisionally sintered green body is cut into a denture shape, followed by sintering it. In such a method, for example, a zirconia powder is subjected to usual press-molding to prepare a green body, and then the green body is provisionally sintered at temperatures of from 700 to 1,000° C., to prepare a mill blank. Then, by CAD/CAM, the prepared mill blank is carved into the shape of the denture, followed by sintering this. For example, the carved mill blank having the shape of the denture is sintered by using a process program such that it is heated to a sintering temperature at a temperature raising rate of 600° C./hr, and the retention time at the sintering temperature is set to be 2 hours, whereby the zirconia is sintered in a short time of about 7 hours.
On the other hand, in sintering by HIP, such sintering is conducted as primary sintering, and it is necessary to conduct HIP as sintering under pressure (secondary sintering), and, in SPS, sintering can be carried out in a short period of time and at a low temperature, but on the other hand, in SPS, a graphite mold is used, whereby heat treatment is required to make a sintered body colored by the material of the mold, to be colorless, by tempering the sintered body. Furthermore, it is necessary to cut a hard zirconia sintered body into the shape of a denture, and for such reasons, these methods are not practically employed. Therefore, it is desired to provide a zirconia powder, whereby a zirconia sintered body having a high density can be prepared by pressureless sintering in a short time.